1. Field of the Invention
The invention relates to integrated circuit devices incorporating programmable logic to provide user programmable output functions, and specifically, devices which provide analog signal outputs.
2. Description of the Related Art
Programmable logic devices (PLD), including the programmable array logic (PAL) device and the programmable gate array, are integrated circuits which can be configured by the user to perform logic functions on digital inputs. PLDs offer digital designers a flexible and cost-effective implementation for complex logic circuits and the best alternative amongst a spectrum of products ranging from fully customized integrated circuits to standard, dedicated-purpose devices.
A typical PAL includes a programmable array of AND gates, and a fixed array of OR gates. In other programmable logic devices, both the AND and the OR arrays are programmable. The outputs of such combinatorial logic arrays used in programmable logic circuits may be coupled directly to an I/O pin, or input to clockable registers. In many devices, the combinatorial logic array outputs are registered and are fed back to the inputs of the combinatorial array. Some programmable logic circuits also include a clockable input synchronizing register located between an input pin and one of the inputs to the combinatorial array. PAL is a trademark of Advanced Micro Devices, Inc. See, Advanced Micro Devices, Inc. PAL.RTM. Device Handbook, (1988).
The programmable logic device has thus given the digital designer a means for reducing circuit size through higher integration, ease of design and documentation by software specification, and the security of keeping one's own design proprietary.
It is desirable to provide a single integrated circuit which has both analog and digital functional components working together to provide an analog output signal responsive to digital inputs which are acted upon by programmable logic functions. Such a device allows great flexibility and programmability by allowing analog outputs to be derived from programmable digital solutions.
One simple solution to this objective is presented in the X9MME E.sup.2 POT Digitally Controlled Potentiometer by Xicor Corporation, Milpitas, Calif., which provides ninety-nine (99) increment, resistor ladder controlled by a digitally decoded counter which includes the capability of storing the count value in non-volatile memory on power down. In addition, several types of digital-to-analog converters are well known in the art, including the "weighted resistor" and the "ladder network" types of DACS. However, to the best of applicants' knowledge, no devices have been provided which provide programmable logic circuitry and a digital-to-analog converter in a integrated circuit device.